The invention relates to an optical fiber network using wavelength division multiplexing (WDM) and an add and drop node for such a network.
Optical multi-channel systems employing wavelength multiplexing are used both in new networks and in order to enhance the transmission capability of existing optical fiber networks. Thus, information channels which previously had to be transmitted on a plurality of separate fiber pairs are forwarded on a single fiber pair in WDM networks. Using optical wavelength division multiplexed channels means that a plurality of serial information signals, i.e. a plurality of serial binary signals, are transmitted on the same optical fiber by modulating such a serial signal on a light signal, also called carrier, having a definite wavelength and then combining the modulated light signals in an optical coupler or optical multiplexer to a composite light signal on the considered optical fiber. The signal primarily modulated on a monochromatic light signal or carrier together with the carrier can be called a channel or traffic channel.
Self-healing optical fiber networks having a ring configuration are disclosed in U.S. Pat. No. 5,442,623, but they are not particularly adapted to WDM-signaling. A similar network designed for WDM-traffic is disclosed in the International patent application PCT/SE98/00136. The networks described in these documents use an extra protecting fiber pair between each pair of nodes.
Optical wavelength multiplexing can generally be used in different optical fiber network configurations or architectures having e.g. only a single fiber pair between a pair of nodes. Such an architecture is the FlexBus(trademark) concept as described in B. S. Johansson et al., xe2x80x9cFlexible bus: A self-restoring optical ADM ring architecturexe2x80x9d, Electronics Letters, Dec. 5, 1996, Vol. 32, No. 25, and U.S. patent application Ser. No. 08/421,734, this architecture comprising a ring configuration of optical links connecting a plurality of nodes. The FlexBus(trademark) concept has emanated from the need for protecting ring networks against fiber cuts and optical amplifier failures, and to solve the problem, often associated with ring network architectures, of circulating signals and noise. In the FlexBus(trademark) architecture one section of the fiber ring is always lade passive or inactive by means of optical switches or amplifiers. This intentionally introduced break effectively eliminates all problems associated with circulating signals and hence allows that less circuit components can be used and circuit elements having lower performance can be used, while still retaining the shortest longest path possible. In the case of a real failure of a link, that link which previously has been intentionally made inactive is made active and the failed link now becomes the inactive link, what can be described by having the inactive link moved from its former position to the failed section. This procedure is called that xe2x80x9cthe bus flexesxe2x80x9d, and thereby the traffic is restored.
In the FlexBus(trademark) channel blocking or selection filters placed in the lines are not needed, which alleviates the problems associated with concatenated filtering. The signal from one transmitter can be sent in both directions simultaneously without causing interference, and the same wavelength can be used in both directions, thus allowing the same number of bi-directional connections to be set up as the number of wavelengths that are used in the network.
With the maturing of filtering and switching technology it would, however, be beneficial to be able to reuse wavelengths more than once in order to be able to set up more connections and thus increase network capacity for the limited number of wavelengths that are feasible in a network with regard to available optical amplifier gain-bandwidth, realistic filter bandwidths and frequency stability of filters and light sources. Thus another implementation of a node architecture, based on the FlexBus(trademark) but including a plurality of blocking filters and switches connected in-line, i.e. in the direct path of a fiber of the network or bus through the node, was invented and is disclosed in the published International patent application WO 96/31025 and is called the xe2x80x9cRearrangeable FlexBus(trademark)xe2x80x9d. That implementation is capable of a very efficient use of the wavelengths. In the published International patent application WO 96/24998 an algorithm scheme for wavelength allocation in Rearrangeable FlexBus(trademark) networks is disclosed. An add/drop node for a WDM network is disclosed in the published International patent application WO 98/49794 which can allow some reuse of wavelengths.
It is an object of the invention to provide an add and drop node for a network of the kind Rearrangeable FlexBus(trademark) as described above having a minimum of in-line components and enhancing the reliability of the network but still having the good properties of the FlexBus(trademark) and the efficient use of wavelengths as defined in the concept of the Rearrangeable FlexBus(trademark).
The problem to be solved by the invention is how to achieve a node construction for a network of the kind Rearrangeable FlexBus(trademark) operating substantially as the nodes of that bus allowing an efficient wavelength allocation in the network and allowing the network and the nodes to operate in a reliable way. In particular, in the network, the nodes should not transmit information over links where it is not needed or where it will not be received by any down-stream node.
Thus generally, an add/drop node is provided which is arranged to be connected in an optical fiber WPM network. The network has a ring configuration including two fibers carrying light signals in opposite directions. Thus, the network has links connecting neighbouring nodes. For protection, the network always has one inactive link carrying no light signals and the network is constructed so that the inactive link can be made active and another link can be made inactive. Since the network is WDM-type, information is carried in the network in a plurality of separate wavelength bands for the light signals. The add/drop node comprises in the conventional way drop couplers and add couplers for each direction for taking out a share of signals at the add/drop node and for adding signals in the add/drop node respectively. The node further comprises as conventional receivers and transmitters for receiving light signals in wavelength bands in the add/drop node and transmitters for transmitting light signals in wavelength bands from the add/drop node into the network. The add/drop node further comprises a band blocking filter arranged between a drop coupler and an add coupler for blocking in one direction all wavelengths which are received and/or terminated in the node from that direction. Preferably, the band blocking filter is arranged for blocking all wavelengths which are received in the node from either direction and are reused in the node.
In the node at least one switch is provided, which advantageously is connected according to one of the following cases:
to a receiver and the drop couplers for allowing the receiver to receive from either one of the two opposite directions, or
to a transmitter and the add couplers for allowing the transmitter to transmit in either one of the two opposite directions.
Preferably, two separate switches are provided, one connected according to the first of said cases and one connected according to the second of said cases.
Further switching means can be provided for letting, in a first position of the switching means, the light signals of a wavelength pass through the node in a substantially unaffected way and in a first direction. The same wavelength is then received from a second direction opposite to the first one. In a second different position of the switching means they block the wavelength as received from the second direction. The switching means for receiving in a wavelength band and at each instant only in one direction can comprise a 2:3 switch having its inputs connected to bandpass filters for the wavelength band. Each bandpass filter is then connected to a drop coupler, so that the switch receives light from opposite directions. The 2:3 switch preferably has one of its outputs connected to a receiver for the wavelength band and another output connected to an add coupler for light signals in a second direction opposite to the first one.
The optical fiber WPM network built from such add/drop nodes and possibly other nodes having the same or corresponding switching facilities allows an efficient use of wavelength. Thus, the nodes can be arranged to use at least one wavelength band in such a way that the wavelength band is used by at least two separate first nodes for transmitting to a second nodes, which are different from each other, in a first direction and is used by only one third node to transmit to a fourth node in a second direction opposite to the first direction.
In the network a transmission span can be defined to be the piece of the network between a node transmitting information in a wavelength band and another node receiving the information on the same wavelength band. Then advantageously, the transmission spans of the at least two separate first nodes for the wavelength band in the first direction are arranged not to overlap each other. The transmission span of the third node for the wavelength in the second direction preferably extends over the spans of the at least two separate first nodes for the wavelength in the first direction.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the methods, processes, instrumentalities and combinations particularly pointed out in the appended claims.